Anterior prosthetic spinal disc replacement

ABSTRACT

The present invention relates generally to a prosthetic spinal disc for replacing a damaged disc between two vertebrae of a spine. The present invention also relates to a method for implanting a prosthetic spinal disc via anterior or anterior lateral implantation. Other surgical approaches for implanting the prosthetic disc may also be used.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/875,782, filed on Oct. 6, 2015 (published as U.S. Pat. Pub. No.2016-0058567), which is a continuation of U.S. application Ser. No.13/434,228, filed Mar. 29, 2012, now U.S. Pat. No. 9,179,925, which is adivisional application of U.S. application Ser. No. 11/246,149, filedOct. 11, 2005, now U.S. Pat. No. 8,167,948, all of which areincorporated by reference herein in their entireties for all purposes.This application is also related to U.S. application Ser. No.10/909,210, filed Jul. 30, 2004, now U.S. Pat. No. 7,641,666, and U.S.application Ser. No. 10/827,642, filed Apr. 20, 2004, now U.S. Pat. No.7,621,956, and to provisional application No. 60/491,271 filed on Jul.31, 2003, all of which are incorporated by reference herein in theirentireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a prosthetic spinal disc for fully orpartially replacing a damaged disc between two vertebrae of a spine. Thepresent invention also relates to a method for implanting a prostheticspinal disc via anterior, anterior-lateral, and lateral implantation,although other implantation approaches may also be used.

BACKGROUND OF THE INVENTION

The vertebral spine is the axis of the skeleton on which a substantialportion of the weight of the body is supported. In humans, the normalspine has seven cervical, twelve thoracic and five lumbar segments. Thelumbar spine sits upon the sacrum, which then attaches to the pelvis,and in turn is supported by the hip and leg bones. The bony vertebralbodies of the spine are separated by intervertebral discs, which act asjoints and allow known degrees of flexion, extension, lateral bending,and axial rotation.

The typical vertebra has a thick anterior bone mass called the vertebralbody, with a neural (vertebral) arch that arises from the posteriorsurface of the vertebral body. The centra of adjacent vertebrae aresupported by intervertebral discs. Each neural arch combines with theposterior surface of the vertebral body and encloses a vertebralforamen. The vertebral foramina of adjacent vertebrae are aligned toform a vertebral canal, through which the spinal sac, cord and nerverootlets pass. The portion of the neural arch which extends posteriorlyand acts to protect the spinal cord's posterior side is known as thelamina. Projecting from the posterior region of the neural arch is thespinous process.

The intervertebral disc primarily serves as a mechanical cushionpermitting controlled motion between vertebral segments of the axialskeleton. The normal disc is a unique, mixed structure, comprised ofthree component tissues: the nucleus pulpous (“nucleus”), the annulusfibrosus (“annulus”) and two vertebral end plates. The two vertebral endplates are composed of thin cartilage overlying a thin layer of hard,cortical bone which attaches to the spongy, richly vascular, cancellousbone of the vertebral body. The end plates thus act to attach adjacentvertebrae to the disc. In other words, a transitional zone is created bythe end plates between the malleable disc and the bony vertebrae.

The annulus of the disc is a tough, outer fibrous ring which bindstogether adjacent vertebrae. The fibrous portion, which is much like alaminated automobile tire, measures about 10 to 15 millimeters in heightand about 15 to 20 millimeters in thickness. The fibers of the annulusconsist of fifteen to twenty overlapping multiple plies, and areinserted into the superior and inferior vertebral bodies at roughly a 40degree angle in both directions. This configuration particularly resiststorsion, as about half of the angulated fibers will tighten when thevertebrae rotates in either direction, relative to each other. Thelaminated plies are less firmly attached to each other.

Immersed within the annulus is the nucleus. The healthy nucleus islargely a gel-like substance having high water content, and like air ina tire, serves to keep the annulus tight yet flexible. The nucleus-gelmoves slightly within the annulus when force is exerted on the adjacentvertebrae while bending, lifting, and other motions.

The spinal disc may be displaced or damaged due to trauma, disease,degenerative defects, or wear over an extended period. A disc herniationoccurs when the annulus fibers are weakened or torn and the inner tissueof the nucleus becomes permanently bulged, distended, or extruded out ofits normal, internal annulus confines. The mass of a herniated or“slipped” nucleus tissue can compress a spinal nerve, resulting in legpain, loss of muscle control, or even paralysis. Alternatively, withdiscal degeneration, the nucleus loses its water binding ability anddeflates, as though the air had been let out of a tire. Subsequently,the height of the nucleus decreases causing the annulus to buckle inareas where the laminated plies are loosely bonded. As these overlappinglaminated plies of the annulus begin to buckle and separate, eithercircumferential or radial annular tears may occur, which may contributeto persistent or disabling back pain. Adjacent, ancillary spinal facetjoints will also be forced into an overriding position, which may createadditional back pain.

Whenever the nucleus tissue is herniated or removed by surgery, the discspace will narrow and may lose much of its normal stability. In manycases, to alleviate back pain from degenerated or herniated discs, thenucleus is removed and the two adjacent vertebrae are surgically fusedtogether. While this treatment alleviates the pain, all discal motion islost in the fused segment. Ultimately, this procedure places a greaterstress on the discs adjacent to the fused segment as they compensate forlack of motion, perhaps leading to premature degeneration of thoseadjacent discs.

As an alternative to vertebral fusion, various prosthetic discs havebeen developed. The first prosthetics embodied a wide variety of ideas,such as ball bearings, springs, metal spikes and other perceived aids.These prosthetics are all made to replace the entire intervertebral discspace and are large and rigid. These devices have met with less thanideal results and improved designs are needed.

While anterior implantation involves numerous risks during surgery,including potential damage to organs during surgery and increased riskto the various blood vessels during surgery, improved designs andmethods of implantation may increase the desirability of anteriorapproaches to prosthetic disc replacements.

A posterior approach to intervertebral disc implantation avoids therisks of damaging body organs and vessels. Despite this advantage, aposterior approach also raises other difficulties that have discouragedit use. For instance, during a posterior approach the spinal cord isexposed and potentially at risk of damage. Additionally, vertebral bodygeometry allows only limited access to the intervertebral discs. Thus,the key to successful posterior or posterior lateral implantation isavoiding contact with the spinal cord, as well as being able to place animplant through a limited special area due to the shape of the vertebralbones.

Accordingly, there exists a need for improved designs for prostheticdiscs. In particular, an improved design for prosthetic discs foranterior implantation are needed. Accordingly, the present inventionprovides improved designs for prosthetic discs implanted into a patientfrom the anterior approach.

SUMMARY OF THE INVENTION

Generally, the present invention is directed toward prosthetic discdesigns. One embodiment of the invention is a prosthetic disc forreplacement of a damaged spinal disc between two vertebrae. Theprosthetic disc has endplates made of rigid material. One plate of rigidmaterial has a surface that can engage with a surface of a vertebralbody. The rigid plate may have a contoured, partially sphericalarticulating surface. A second rigid plate having a second surfaceengages with the surface of a second vertebral body, and has acontoured, partially cylindrical articulating surface. A core elementmay be at least partially disposed between the first and second rigidplates. Moreover, the core element may have contoured surfaces incommunication with and substantially corresponding to the curvature ofthe first and second rigid plate articulating surfaces.

In one embodiment, one or both of the rigid plates are configured tocorrespond to the natural curvature and shape of the vertebral bodyendplates. In another embodiment, however, one or both of the rigidplates are configured to have a preselected shape or contour. Thus, thesurface of the vertebral body that contacts the rigid plate may beshaped or prepared for receiving or mating with the preselected shape orcontour of the one or more rigid plates. In one example embodiment, theportion of one or both plates that contacts a vertebral body issubstantially flat.

In another embodiment, the prosthetic disc is formed from a plurality ofassemblies. The first assembly comprises the first rigid plate, secondrigid plate, and core element. The second assembly comprises a thirdrigid plate configured and adapted to engage with the first endplate ofthe first vertebral body, and has a contoured, partially sphericalarticulating surface having substantially the same radius of curvatureas the first rigid plate articulating surface. The second assembly alsomay have a fourth rigid plate configured and adapted to engage with thesecond endplate of the second vertebral body, and having a contoured,partially cylindrical articulating surface having substantially the sameradius of curvature as the second rigid plate articulating surface.Likewise, the second assembly may have a second core element at leastpartially disposed between the third and fourth rigid plates, whereinthe second core element has a contoured surfaces substantiallycorresponding to the curvature of the third and fourth rigid platearticulating surfaces.

Several embodiments of the present invention are directed toward anartificial disc that is capable of providing a moving instantaneous axisof rotation OAR). In one embodiment, the moving IAR achieved issubstantially in the sagittal plane.

In many embodiments, the contact between the first rigid platearticulating surface and the first contoured surface of the first coreelement extends over an area. Likewise, the second rigid platearticulating surface and the second contoured surface of the first coreelement may also extend over an area. While it is preferred that boththe first and second articulating surfaces contact the core element overan area, one or both surfaces may be configured to contact the coreelement along a line or a point. For instance, in one embodiment thecontact between the second rigid plate articulating surface and thesecond contoured surface of the first core element forms a line ofcontact.

In some embodiments, the orientation or relative position of thearticulating surfaces may be specified. For example, in one embodiment,the first rigid plate is disposed above the first core element and thesecond rigid plate is disposed below the first core element.

In another embodiment, one or both rigid plates may have a keel orraised ridge of material that extends at least partially into theendplate of the vertebral body that they contact.

A variety of materials may be used to form the components of theinvention. For instance, in one embodiment the first core element is atleast partially formed of an elastomeric material.

The artificial disc may also have mechanical stops that limit movementof the disc. For example, stops may be provided to prevent lateralbending greater than 10 degrees in each direction. In addition,mechanical stops may prevent total axial rotation greater than 5 to 10degrees.

The curvature of the articulating surfaces may be convex or concave. Inone embodiment, the curvature of the second rigid plate articulatingsurface is convex. In another embodiment, the curvature of the secondrigid plate articulating surface is concave. The dimensions of eachcomponent also may be varied. For example, in one embodiment the firstrigid plate may have a length from about 10 to about 45 mm, while inanother embodiment the first rigid plate may have a length from about 22to about 26 mm. In yet another embodiment, the first rigid plate mayhave a width from about 7 mm to about 36 mm, or alternatively may befrom about 7 mm to about 15 mm. In still another embodiment, the firstrigid plate has a width from about 8 to about 12 mm, and in a furtherembodiment the first rigid plate has a width from about 12 mm to about36 mm. Moreover, other embodiments the first rigid plate may have awidth from about 16 mm to about 28 mm, or from about 12 to abut 14 mm.

In one embodiment the core element and endplates are formed fromsubstantially similar materials, while in another embodiment the coreelement is formed from a different material that the endplates. In oneembodiment, the core comprises a high molecular weight polymericmaterial, and more specifically may comprise a high molecular weightpolyethylene. The core may also be formed from polyetheretherketone(PEEK) or other radio translucent materials. In embodiments where radiotranslucent materials are used, the core may have a radio opaque markerthat is capable of indicating the orientation of the core. For example,the radio opaque marker may be two or more metallic pins withorientations that permit identification of the orientation of the core.Additionally, all components of the prosthetic disc may be made out ofmetal. Preferably, when metal is used as the material to make theprosthetic disc, a biocompatible metal, such as cobalt-chromium ortitanium, is used.

Methods for replacing a damaged spinal disc between two vertebrae andtools and/or instruments thereof are also contemplated by the presentinvention. One embodiment involves the steps of removing a damagedspinal disc disposed between two vertebral bodies, providing andpositioning a first artificial disc assembly therebetween. In someinstances, one or both endplates of the vertebral bodies may be preparedfor receiving the artificial disc.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood with reference to theembodiments thereof illustrated in the attached figures, in which:

FIG. 1 is a side-view of an embodiment of a prosthetic disc design ofthe present invention;

FIG. 2 is a cross sectional view of an embodiment of a prosthetic discdesign of the present invention.

FIG. 3 is a cross sectional view of an embodiment of a prosthetic discdesign of the present invention.

FIG. 4 is an medial-lateral view of an embodiment of a prosthetic discdesign of the present invention;

FIG. 5 is an anterior-posterior view of the embodiment of FIG. 4;

FIG. 6 is an medial-lateral view of an embodiment of a prosthetic discdesign of the present invention;

FIG. 7 is an anterior-posterior view of the embodiment of FIG. 6;

FIG. 8 is an illustration of an embodiment of the prosthetic disc designof the present invention;

FIG. 9 is an illustration of an embodiment of the prosthetic disc designof the present invention;

FIG. 10 is an illustration of an embodiment of the prosthetic discdesign of the present invention;

FIG. 11 is an illustration of an embodiment of the prosthetic discdesign of the present invention;

FIG. 12 is an illustration of one embodiment of a central core member;

FIG. 13 is an illustration of an embodiment of the prosthetic discdesign of the present invention;

FIG. 14 is an illustration of a core member and bottom endplate in anembodiment of the present invention;

FIG. 15 is an illustration of a core member and bottom endplate in anembodiment of the present invention;

FIG. 16 is an illustration of an embodiment of the prosthetic discdesign of the present invention;

FIG. 17 is an illustration of an embodiment of the prosthetic discdesign of the present invention;

FIG. 18 is an illustration of an embodiment of the prosthetic discdesign of the present invention;

FIG. 19 is an illustration of an embodiment of the prosthetic discdesign of the present invention;

FIG. 20 is an illustration of a core member and bottom endplate in anembodiment of the present invention;

FIG. 21 is an illustration of a core member of an embodiment of thepresent invention;

FIG. 22 is an illustration of a core member of an embodiment of thepresent invention;

FIG. 23 is an illustration of an endplate of an embodiment of thepresent invention;

FIG. 24 is an illustration of an endplate of an embodiment of thepresent invention;

FIG. 24A is an illustration of a core member of an embodiment of thepresent invention;

FIG. 25 is an illustration of an endplate of an embodiment of thepresent invention;

FIG. 26 is an illustration of an endplate of an embodiment of thepresent invention; and

FIG. 27 is an illustration of an endplate of an embodiment of thepresent invention.

DETAILED DESCRIPTION

The present invention relates generally to a prosthetic spinal disc forreplacing a damaged disc between two vertebrae of a spine. The presentinvention also relates to a method for implanting a prosthetic spinaldisc via anterior implantation. In particular, the present inventionencompasses a method for implanting the prosthetic spinal disc via ananterior approach.

As described in detail below, the prosthetic spinal disc may bearticulating or non-articulating.

Several embodiments of the invention illustrate different examples ofhow the interfacing surfaces of an articulating prosthetic disc may beformed. For instance, articulation may be accomplished with oneinterfacing surface, such as a ball and joint, or alternatively may beaccomplished with two or more interfacing surfaces such as a coredisposed between an upper and lower articulating surface. Theconfiguration of the surface contact may vary to permit or restrictdifferent types and ranges of motion of the treated area. Thus, thecontact profile of the interfacing surface may be an area (such as witha ball and socket configuration), a line (such as with a roller orsleeve bearing), or a point (such as with a ball bearing).

The materials used for different embodiments of the invention willdepend to some extent upon the type of surface contact being used aswell as the type and extent of wear that may result. Examples ofmaterials that may be used include, but are not limited to polyethylene(or other elastomeric material) on metal, metal on metal, or ceramic onceramic.

The present invention also allows for customization of the instantaneousaxis of rotation (IAR) and/or the center of rotation (COR) of onevertebral body with reference to another. The IAR and COR of a healthyvertebral body with respect to another is constantly changing in allplanes because of pushing, pulling, and tethering of the segment throughits range of motion by the ligaments, annulus, muscles, facets and otherportions of the spine. Often, prosthetic disc replacement designs failto mimic this varying IAR and COR. For example, a fixed ball and sockethas a fixed IAR and COR. One potentially adverse result from using aprosthetic disc having a constrained implant is that the device maycause damage to facet joints due to anatomical interferences that mayoccur with a fixed axis of rotation. On the other hand, in generalconstrained IAR systems have been more stable than past designsutilizing a moving IAR. One example of a prosthetic disc having a fixedIAR is described in U.S. Pat. No. 5,314,477.

Conversely, past devices utilizing a moving IAR have provided theadvantage of allowing for shear translation and of at least partiallymimicking of the moving IAR of a healthy spine. These advantages,however, typically have been achieved in the past at the expense of aloss of stability of the device. Some examples of prosthetic discdesigns having a moving IAR are described in U.S. Pat. Nos. 4,759,766,5,401,269, and 6,414,551.

In contrast, the present invention allows for an implant design that canmimic or partially mimic this varying IAR and COR to the extent desiredby a physician while also preserving stability of the device. Forexample, one embodiment of the invention is a prosthetic disc thatprovides a moving IAR substantially in the sagittal plane so that apatient can more easily flex and extend the spine while limiting themovement of the IAR under lateral bending. It is believed that thisconfiguration provides the best of both worlds by allowing a moving IARfor the predominant or more common motion a patient may undertake inday-to-day life while limiting lateral bending to achieve greaterstability to the device. Several embodiments of the invention permittranslation of one vertebral body with respect to another. By allowingfor translation in the transverse plane, the disc designs results in theIAR and COR also translating in the transverse plane. As explainedfurther below, one additional way of achieving a varying IAR and/or CORin three dimensional space is by combining two articulating surfacesopposing one another that can both translate with respect to each otherin the transverse plane and rotate in other planes.

The interfacing surfaces of articulating and non-articulatingembodiments of the present invention also allow for varying degrees ofrotational and linear translation, and several embodiments of thepresent invention likewise permit a similar range of rotation and lineartranslation. Rotational translation is the movement of theintervertebral segment as a result of movement such as flexion,extension, and lateral bending. There are two components in thistranslation: one in the anterior/posterior direction and one in thetransverse plane. Linear translation is translation in the transverseplane as a result of shear forces applied to the intervertebral segment.Thus, a ball and socket mechanism fixed in one location relative to theintervertebral segment would allow only rotational translation but wouldnot permit linear translation. As illustrated in many of the embodimentsthat follow, however, linear translation of a ball and socketconfiguration could be achieved if the ball and socket were able to movein the transverse plane.

The present invention also contemplates the use of a prosthetic spinalimplant that has a fixed IAR. For example, in one embodiment of thepresent invention comprises a two piece assembly design. Each assemblyis comprised of a top and bottom endplate with interior surfaces thatare curved. The surfaces contact each other and allow for rotation aboutthe longitudinal axis of the spine as well as bending across the axialplane.

Endplates are used to associate the prosthetic disc with the vertebralbodies neighboring the disc. The endplates may be configured in severalways to help ensure a desired endplate-bone interface. For instance, theendplates may have one or more keels that extend into the bony portionof the vertebral body. Over time, bony ongrowth/ingrowth will surroundthe endplate and further help secure the endplate to the vertebral body.

In addition to keels, the endplate may have other or additional geometrythat helps securely hold the endplate in place. For example, the endplate may have one or more teeth, rails, ribs, flanges, or otherconfigurations that can help provide a surface that can secure theendplate more readily to the bone. Other short-term fixation may includescrews or other fasteners that hold the end plate to the vertebral body.In some embodiments, these fasteners may be removed once a morelong-term interface has been established, or alternatively the fastenersmay remain in place indefinitely or until the prosthetic disc needsadjustment and/or replacement.

In addition to providing an endplate surface geometry or configurationthat may promote bony ongrowth/ingrowth to hold the interfacing surfacestogether securely over the long term, these configurations also may helpprovide short term fixation of the endplate to the vertebral body. Forexample, a keel may have a wedge shape so that the width of a first endof the keel near the endplate is narrower than the width of the distalend. Once installed, the inverted wedge of the keel helps preventseparation of the endplate from the vertebral body at least until bonyongrowth/ingrowth can more securely hold the endplate in place.

To help accelerate and to further promote bony ongrowth/ingrowth at theinterface between the vertebral body and the end plate, the end platemay be coated with an osteoconductive material and/or have a porous ormacrotexture surface. For example, the end plate may be treated with acoating that promotes bone growth. Examples of such coatings include,without limitation, hydroxyl appetite coatings, titanium plasma sprays,sintered beads, or titanium porous coatings.

FIG. 1 is a side view of a prosthetic spinal disc 1 which may be locatedbetween sequentially aligned vertebral bodies, such as are found in thecervical, thoracic, and lumbar spine. Prosthetic spinal disc 1 conformsin size and shape with the spinal disc that it replaces and restoresdisc height and the natural curvature of the spine. Prosthetic spinaldisc 1 comprises two opposite end plate 5 and 7 which are disposed intwo substantially parallel horizontal planes when it is at rest, i.e.,when it is not subjected to any load, either moderate or heavy.

Outer faces 6 and 8 of end plates 5 and 7 are in direct contact withvertebral bodies (not shown) and may be textured or have a plurality ofteeth to ensure sufficient contact and anchoring to the vertebralbodies. Outer faces 6 and 8 of end plates 5 and 7 may also have a porousor macrotexture surface that facilitates bone ongrowth/ingrowth so thatthe prosthetic spinal disc 1 is firmly affixed to vertebral bodies.Outer faces 6 and 8 of end plates 5 and 7 may also have keels 9 and 10that may fit into channels in the vertebral bodies to facilitateanchoring. Disposed between the inner faces 11 and 12 of end plates 5and 7 is a core 13, which is securely placed between the inner faces ofend plates 5 and 7. A stop member 15 is formed around the equator of thecore 13, which functions to limit the motion of vertebral bodies beyonda predetermined limit that is deemed unsafe to the patient.

As shown in FIG. 1, mechanical stops may be designed as part of theupper and lower endplates 5 and 7 and core 13. In this embodiment, themechanical stop results as a function of the design of the upper andlower endplates 5 and 7 and the design of the core 13. For example andwith continuing reference to FIG. 1, upper endplate 5 is designed with afirst contact surface 14 on the interior surface 11 of the upperendplate 5. Core 13 is designed with a second upper contact surface 15.During flexion and/or extension, first contact surface 14 may abut orcome into contact with second contact surface 15, thus effectivelylimiting the degree of movement allowed by the articulating surfaces.Similarly, the core is designed with a lower third contact surface 16while lower endplate is configured with an upper fourth contact surface17. During extension and/or flexion, third contact surface 16 may abutor come into contact with fourth contact surface 17, thus effectivelylimiting the degree of movement allowed by the articulating surfaces.Contact surfaces 14, 15, 16, and 17 may be formed with various slopes,angled surfaces, and thickness to specify the degree of movementpermitted by the disc design.

In an alternative embodiment, the stop member may be formed from a ridgeof material disposed about the partially spherical surface of the coreabove the equatorial plane of core 13. As seen in FIGS. 2 and 3, a crosssectional view of an embodiment of the prosthetic disc is shown.Prosthetic disc 1 has an upper endplate 5, lower endplate 7 and coremember 13 disposed there between. As seen in FIGS. 2 and 3, core memberis formed with a lip or rim 19 that serves as a stop member. As endplate 5 moves relative to core 13 in response to movement of the spine,stop member 19 may approach or engage with end plate 5 to restrictfurther motion in a particular direction. Endplate 5 may be configuredwith a raised edge that mates or abuts with the stop member 19 of coremember 13. Stop member 19 may be formed of a relatively rigid materialso that additional motion is substantially prevented once engagedagainst an end plate. Alternatively, the stop material may be made ofresilient material that provides some cushioning or flex fromdeformation of the stop material before the range of motion is fullylimited.

In alternative embodiments, stop member 19 may be disposed on one ormore of end plates 5 and 7. For instance, end plates 5 and 7 may beconfigured with raised areas or ridges on its perimeter that engage witheither core 13 or an opposing end plate in order to limit further motionin a particular direction. Any of the stop members discussed above maybe designed to limit motion to a greater degree in one direction than inanother. Thus, the stop member may have various shapes and thicknessesto provide a variable range of motion that favors or disfavors movementin particular planes. For example, the stop member may have increasedthickness on the side portion of the core to provide a more limitedrange of lateral motion of the spine while still allowing motion in theposterior/anterior direction.

The motion segment comprises a anterior prosthetic spinal disc 1 andadjacent upper and lower vertebral bodies. The exact contours of core13, inner surfaces of endplates 5 and 7 and stop member 19 determine therange of motion allowed in flexion and extension, side bending, shearand rotation.

It is preferred that anterior prosthetic spinal disc 1 closely mimicsthe mechanical functioning and the various physical attributes of thenatural spinal disc that it replaces. In some instances, however, theprosthetic spinal disc may permit a more limited range of motion in oneor more directions in order to prevent further spinal injury. Ingeneral, the prosthetic spinal disc can help maintain the properintervertebral spacing, allow for proper range of motion, and providegreater stability. It can also help transmit physiological stress moreaccurately.

End plates 5 and 7, core 13, and stop 19 may be composed of a variety ofbiocompatible materials, including metals, ceramic materials andpolymers. Such materials include, but are not limited to, aluminum,cobalt-chromium, alloys, and polyethylene. Outer surfaces 6 and 8 of theend plates 5 and 7 may also contain a plurality of teeth, maybe coatedwith an osteoconductive material, antibiotics or other medicament, ormay have a porous or macrotexture surface to help rigidly attach the endplates to the vertebral bodies by promoting the formation of new bonyongrowth/ingrowth. Such materials and features may be used in any of theanterior prosthetic spinal discs described herein.

FIGS. 4-8 illustrate various embodiments of the invention includingcontacting surfaces that complement each other to form an arced orcurved surface in the medial-lateral direction and in theanterior-posterior direction. FIG. 4 illustrates the curvature createdin the medial-lateral direction (designated L-M-L), while FIG. 5 showsthe curvature created in the anterior-posterior (designated A-L)direction. As shown in FIG. 4, first endplate 50 has an outer surface 52that contacts a vertebral body (not shown). Second endplate 60 has anouter surface 62 that contacts a vertebral body (not shown). The outersurfaces of first endplate 50 and second endplate 60 are configured withkeels 54 and 64 respectively, that may be inserted into the vertebralbodies. FIG. 4, shows the inner surfaces 56, 66 of the first and secondendplates respectively configured to create a curve. When viewed in themedial-lateral direction, the curved surfaces 56, 66 of the first andsecond endplates 50 and 60 allow the vertebral bodies (not shown) tomove relative to each other in the medial-lateral direction. As shown inFIG. 5, first endplate 50 has an outer surface 52 that contacts avertebral body (not shown). Second endplate 60 has an outer surface 62that contacts a vertebral body (not shown). The outer surfaces of firstendplate 50 and second endplate 60 are configured with keels 54 and 64respectively, that may be inserted into the vertebral bodies. FIG. 5,shows inner surfaces 56, 66 of the first and second endplates 50 and 60respectively configured to create a curve. When viewed in theposterior-anterior direction, the curved surfaces 56, 66 of the firstand second endplate allow the vertebral bodies (not shown) to moverelative to each other in the posterior-anterior direction.

The implants may be configured according to a variety of factors such asthe size of the vertebral bodies, the loading that the implants willundergo, and the range of motion desired. Also, while permittingmovement to mimic or partially mimic the natural movement of the spine,it may be desirable that prosthetic discs be designed to maintain adistance between the vertebral bodies that approximate the height of anatural disc. These design considerations may be accomplished in anynumber of ways. For example, one aspect of the design of the implantsthat may vary is the radius of curvature. The radius of curvature may bevaried between implant designs to accommodate different ranges ofmotion. For example, changes in the design of the radius of curvature ofthe articulating surfaces of the endplates allow the designer of theimplant to vary the range and/or type of motion the implant will allow.In other embodiments, the height of the implants may be varied.Accordingly, changes in the thickness of one or more of the endplatesallow the designer of the prosthetic disc to accommodate differentspacing between the two vertebral bodies. Preferably, the height of thesuperior or upper endplate is designed to provide the height preferencesof any particular embodiment. Accordingly, a surgeon may be providedwith a kit comprising one inferior endplate and several superiorendplates. While the articulating surfaces of each superior endplatefits with the inferior endplate, the surgeon may select from varioussuperior endplates of differing thickness to tailor the implantedprosthetic disc to approximate each individual's own ideal or naturaldisc height. In disc designs comprising a three component design,namely, two endplates and a central core member, alternative embodimentscontemplate providing a surgeon with endplates of fixed height andvarious core members of differing heights. Thus, the surgeon may selecta core member to achieve the desired spacing once the disc is implantedinto a patient.

Referring to FIGS. 6 and 7, which are similar in orientation to FIGS. 4and 5, the upper and/or lower portions of the implants may have stops tohelp limit motion in one or more directions. As shown in FIG. 6, forexample, medial-lateral movement can be controlled or limited byincluding stops 58 and 59 on both sides of lower surfaces 56 of endplate50. As shown in FIG. 6, the opposing surfaces 68 and 69 of endplate 60is designed to engage stops 68 and 69, preventing further movement inthe direction restricted. Alternatively, a resilient material may bedisposed between stop 58 and 59 and opposing surfaces 68 and 69 in orderto provide cushioning and to allow resistance to further movement toincrease progressively. FIG. 7 illustrates that stops 58 and 59 may besimilarly used on one or more sides of the implant to limit the range ofmotion in the anterior-posterior direction. While the stops in FIGS. 6and 7 are illustrated protruding upwards or downwards, otherconfigurations also may be used to create a stop or to limit motion. Forinstance, the sliding surface of the portions of the implants may beprevented from further movement simply by contacting the end plate ofthe opposing portion.

FIG. 8 illustrates one embodiment of the invention where differentsurfaces of the prosthetic disc provide for different types of movement.In FIG. 8, prosthetic disc 70 comprises two assemblies 71 and 72, eachassembly comprised of endplates 73 and 74, and a core member 75 disposedbetween the endplates 73 and 74. In this embodiment of the presentinvention, the articulating surfaces between upper endplates 73 andupper surfaces of the core member 75 are configured to permit onlylateral bending. The radius of curvature of the articulating surfaces isdesigned to provide a uniform contact area between the upper endplates73 and the core members 75, with the dimensions of the radius beingdependent on the final spacing of the two assemblies 71 and 72. Thearticulating surfaces between the lower endplates 74 and lower surfaceof the core member 75, are configured to with a ball and socketinterfacing configuration or radiused rail configuration that allows foraxial rotation.

One example of the present invention is illustrated in FIGS. 9-11. Inthis embodiment of the present invention, a prosthetic disc configuredwith a fixed IAR is shown. The prosthetic disc is comprised of twoendplates 80 and 90. In this embodiment the endplates are shown withoutthe keels previously described, although in alternative embodiments,they may be present. First endplate 80 has an upper surface 81 and lowersurface 82. Second endplate 90 has lower surface 91 and upper surface92. In this embodiment, lower surface 82 of first endplate 80 directlyengages upper surface 92 of second endplate 90. Lower surface 82 andupper surface 92 of endplates 80 and 90 are configured such that theyare substantially spherical in curvature and have substantially the sameradius of curvature. This design allows the interacting surfaces tocontact over an area as opposed to a line or point.

While the discussion relating to FIGS. 4-7 relate to two componentdesigns, i.e. an upper and lower endplate with articulating surfaces, itwould be apparent to one of skill in the art that each of the precedingdesigns may be comprised of a three component design, i.e. with a coreelement disposed between the upper and lower endplates.

Accordingly, prosthetic disc designs of the present invention may alsocomprise three component parts: an upper rigid plate, a lower rigidplate, and a central core or core element. The core element is disposedgenerally between articulating surfaces of the upper and lower plates.The articulating surfaces of each plate may be contoured to provide adesired range of motion. For example, one or more of the articulatingsurfaces may have a substantially spherical curvature. In this manner,the articulating surface may generally correspond to a portion of a ballor a socket. The central element may likewise have a contoured surfacethat generally has the same curvature as the articulating surface itcontacts. Thus, a spherical-shaped articulating surface can receive orcontact a portion of the central element having a spherical contourhaving a similar radius of curvature. The contact between the twosurfaces may therefore correspond to a portion of a ball and socket.

Providing a spherical surface allows the two components to rotate andslide across the contacting surfaces in a manner that would permitbending and rotation of one vertebral body relative to another. If thesetwo contacting surfaces were the only elements allowing movement, theIAR of the disc would be fixed. In some embodiments, a second contactingsurface is provided. The second contacting surface allows the disc tomimic a variable IAR of a healthy disc. For example, a second contactingsurface between the second rigid plate and the central element may havea cylindrical contour, preferably allowing the core element to providerotation in the anterior-posterior direction. Thus, it is preferred thatthe cylindrical surfaces of the second rigid plate and core element havean axis of rotation that extends approximately in a lateral direction.

The combination of a spherical shaped surface contact between one plateand a portion of the core element with a second generally cylindricalcontacting surface between another plate and another portion of the coreelement allows the disc to have a variable IAR. This configuration alsoallows for translation of one vertebral body relative to anothervertebral body without requiring either vertebral body to rotate andwithout requiring the distance between the vertebral bodies to increaseor decrease.

The curvature of the articulating surfaces of the plates may be concaveand the corresponding contoured portions of the core element may beconvex to provide contact between the surfaces. For example, in oneembodiment as shown in FIG. 12 core element 100 may have a contouredconvex surface 102 that it semi-spherical or generally corresponds to aportion of a spherical surface, and a contoured convex surface 104 thatis semi-cylindrical or generally corresponds to a portion of a cylinder.Alternatively, one or more of the contoured surfaces of the core elementmay be concave and the articulating surface for which it engageslikewise may be inverted. For example, in one embodiment core elementmay have a contoured convex surface that it semi-spherical or generallycorresponds to a portion of a spherical surface, and a contoured concavesurface that is semi-cylindrical or generally corresponds to a portionof a cylinder. One advantage of this configuration is that it may becapable of achieving a lower overall height than a core element havingtwo convex contoured surfaces.

It is preferred that the contact between the articulating surface of aplate and a contoured surface of a core element extends over an arearather than a line or a point. More preferably, all contact surfaces ofthe invention extend over an area. However, if a convex semi-sphericalsurface were formed with a smaller radius of curvature than thecorresponding concave surface, it would be possible to have the contactbetween the two surfaces correspond to a point contact. Likewise, aconvex cylindrical surface may be formed to be smaller than the concavecylindrical surface it engages with in order to form a contact surfacecorresponding to a line.

The following examples further illustrate how several of the featuresdescribed above may be implemented in a prosthetic disc.

In one embodiment of the present invention, a three component prostheticdisc is provided. As seen in FIG. 13, the prosthetic disc is comprisedof a first endplate 110, a second endplate 120, and a central core 130.First endplate 110 has upper surface 112 and lower surface 114. Thesecond endplate 120 has upper surface 122 and lower surface 124. Centralcore 130 has upper surface 132 and a lower surface 134. While thisparticular embodiment is described with surfaces that have certainorientations, it should be understood that the surfaces described may beplaced on an upper or lower component and that the invention is notrestricted or limited to the orientations described.

The components may be designed with interacting surfaces that allow fordifferent types of movement. For example, the lower surface of the firstendplate and upper surface of the central core may be designed assubstantially spherical surfaces. The interaction between the lowersurface of the first endplate and the upper surface of the central core,thus occurs over an area. The interacting surfaces allow the surfaces tomove with respect to each other in three directions: rotationally,medial-laterally, and in the anterior posterior direction. Theinteraction between the upper surface of the second endplate and thelower surface of the central core may be different. For example, theupper surface of the second endplate and the lower surface of thecentral core may be substantially cylindrical in shape. The twocylindrical surfaces permit rotational movement essentially in onedirection (i.e. about one axis). Preferably, the radii of curvature ofboth cylindrical shapes are approximately the same such that the surfacecontact is over an area instead of a line. In this manner, thecylindrical surface can be configured to permit bending whilerestricting rotation. Thus, during flexion or extension both interfacesurfaces permit movement and result in a moving IAR.

In an alternative embodiment, the contacting surfaces may be designed tolimit movement. For example, the upper surface of the core element andlower surface of the endplate may be substantially spherical in shapeyet only allow rotational movement. In this embodiment of the presentinvention, the endplates and core elements are designed to limit thefirst contacting surfaces from moving in the medial lateral direction oranterior posterior direction.

In an alternative embodiment, a second cylindrical interfacing surfacecan be substituted for the spherical surface. This second cylindricalinterfacing surface may be disposed orthogonally to the direction of thefirst cylindrical interfacing surface. In this manner, one surface willpermit motion in one direction, such as flexion and extension, while thesecond will permit lateral bending.

FIGS. 13-19 illustrate the types of motion that may be achieved using afirst interfacing surface that is generally spherical and a secondinterfacing surface that is generally cylindrical. FIG. 13 illustrates adisc disposed in a neutral position having a disc height H. Theprosthetic disc of the present embodiment is capable of mimicking orpartially mimicking natural movement. For example, FIG. 14 shows lowerendplate 120 and central core member 130 of an anterior prosthetic discduring extension. As can be seen in FIG. 13, articulation between uppersurface 122 of lower endplate 120 and lower surface 134 of central core130, allow for extension of the spine. As the articulating surfaces,i.e. upper surface 122 of lower endplate 120 and lower surface 134 ofcore element 130, in this embodiment are partially cylindrical in shape,the rotation allowed by articulating surfaces 122 and 134 allow rotationsubstantially in the posterior-anterior direction only. FIG. 15 showsthe relative position of the central core 130 and lower endplate 120,during the opposite motion shown in FIG. 14, i.e. flexion.

In addition to the rotation allowed by the partially cylindricalarticulating surfaces between the lower endplate and central core memberin the posterior-anterior direction, the present embodiment also allowsfor translation in the anterior-posterior direction. As shown in FIGS.16 and 17, the present embodiment is capable of translation duringflexion. For example, FIG. 16 shows the relative position of the threecomponents, i.e. upper endplate 110, lower endplate 120, and centralcore member 130, of a prosthetic disc during flexion. As can be seen inFIG. 16 and as previously described with respect to FIGS. 14 and 15,partially cylindrical upper surface 122 of lower endplate 120 and lowersurface 134 of the central core member 130, allow the vertebral bodies(not shown) to rotate substantially in the anterior-posterior direction.In FIG. 16, rotation is in the anterior direction relative to theprosthetic disc's neutral position (shown in FIG. 13). FIG. 17illustrates the relative positions of the three components of aprosthetic disc, i.e. upper endplate 110, lower endplate 120, andcentral core member 130, during both flexion and translation. As can beseen in FIG. 17, the prosthetic disc is in flexion, i.e. rotation in theanterior direction along the upper surface 122 of the lower endplate 120and lower surface 134 of central core member 130. In addition torotation in the anterior-posterior direction, FIG. 17 shows the relativeposition of the components of the prosthetic disc during translation. AsFIG. 17 illustrates, partially spherical surface 114 of upper endplate110 and partially spherical surface 132 of core member 130 are designedto allow upper endplate 110 to translate with respect to central coremember 130. FIG. 17 illustrates the relative position of the componentsof a prosthetic disc during translation in the posterior direction.Because endplates 110 and 120 are attached to vertebral bodies, theranges of motion capable by the prosthetic disc allow the vertebralbodies to move in the same direction, hence, mimicking or partiallymimicking natural disc movement.

FIGS. 18 and 19 simply further illustrate the relative positions of thecomponents of a prosthetic disc during both rotation and translation inthe anterior-posterior direction during extension. Similar to thedescription provided with respect to FIGS. 16 and 17, FIG. 18illustrates how partially cylindrical surfaces 122 and 134 of the lowerendplate 120 and central core member 130, respectively, allow forextension, i.e. rotation in the posterior direction. And similar to thedescription provided with respect to FIGS. 16 and 17, FIG. 19illustrates how partially spherical surfaces 114 and 132 of upperendplate 110 and central core member 130, respectively, allow fortranslation, i.e. substantially linear movement in the axial plane. FIG.19, in particular, illustrates translation in the anterior directionduring extension.

In the embodiments shown in FIGS. 13-19, the third major movementallowed by the prosthetic disc design is rotation in the axial plane. Asdescribed previously, lower surface 114 of upper endplate 110 and uppersurface 132 of core member 130 are partially spherical. Accordingly, thearticulating surfaces allow for axial rotation, in conjunction with thelateral bending and translation provided by the prosthetic disc designs.

The various embodiments of the present invention may have limits onmotion or may not. In non-limited motion designs, the prosthetic discdesign does not limit the range of movement. Rather, in these designs,the various components of the vertebra, including vertebral bodies,muscles, ligaments, facet joints, and other elements of the body limitthe movement of the components of the prosthetic discs. In limitedmotion designs, mechanical stops are provided to limit the range ofmovement of the components of the prosthetic disc. The stops may bedesigned to limit one, two, or more of the various types of movementscapable by the prosthetic discs.

For example in one embodiment of the present invention, stops may beprovided to limit rotation in the anterior-posterior direction. Withreference to FIGS. 16 and 18, core member 130 may be formed with a ringof material designed to interact with endplates 110 and 120 of theprosthetic disc. As seen in FIG. 16, for example, core element 130 isformed with stop 135 that interacts with interacting surfaces 125 and126 of lower endplate 120 of the prosthetic disc. Accordingly, duringflexion as seen in FIG. 16, the interaction between stop 135 andinteracting surface 126 of lower endplate 120 limits the amount ofrotation that may occur between articulating surface 122 of endplate 120and surface 134 of core 130. Similarly, FIG. 18 illustrates theinteraction between stop 135 and interacting surface 125 of lowerendplate 120 during extension.

Either or both stop 135 or interacting surfaces 125 and 126 of lowerendplate 120 may be configured to provide for limits of motion. Forexample, in one embodiment the stop is configured to limit rotation ofthe partially cylindrical surfaces of the lower endplate and core membersuch that rotation may be greater in one direction, i.e. during flexion,than during extension. Alternatively, as described previously, stopand/or interacting surfaces of the lower endplate may be configured withsloped or angled surfaces of varying degrees to precisely control therange of movement. For example and as seen in FIG. 20, stop 135 isconfigured with an angled surface 136 of 5.degree. The opposite,interacting surface 126 of endplate 120 is similarly configured with anangled surface of 15.degree. It would be apparent to one of skill in theart that either one or both of these angled surfaces may be varied toboth create a mating surface upon rotation and control the range ofmotion allowed by the articulating surfaces. And as discussedpreviously, either or both the stop or interacting surfaces of the lowerendplate may have different angles or thickness to provide varyingcontrol of motion. Hence, one prosthetic disc design may allow greaterdegrees of motion during flexion than extension. Alternatively, otherdisc designs may allow greater degrees of motion during extension thanflexion. While any number of variations may be employed within the scopeof the present invention, preferably the limitations of flexion andextension approximate the ranges of motion of a normal, healthy disc.

In addition to stops that limit rotation in the lateral plane, theprosthetic discs of the present invention may also provide stops toconstrain translational movement. For example, ring 135 about coremember 130 may be designed with a lip or rim 137 that rises in thesuperior direction about the circumference of stop 135 of core member130. Similarly, upper endplate 110 may be designed with an interactingsurface 115 configured to contact the inner portion 138 of lip 137. Asseen in both FIGS. 17 and 19, during translation, inner surface 138 oflip 137 contacts interacting surface 115 of upper endplate 110, thus,limiting translation of the components of the prosthetic disc. Again,while the present invention is not limited to any particular ranges ofmovement, and in fact contemplates unconstrained devices, preferably astop is provided to limit translation of the prosthetic disc componentsto approximate the range of motion naturally occurring in a normal,healthy disc.

The stops of the present invention may also be used to prevent thecentral core member from slipping out of alignment with the upper andlower endplates. Accordingly, lip 137 of core member 130, may preventcore member 130 from coming out of alignment with endplates 110 and 120.Additional stops may be provided that further help prevent the coremember from slipping out of alignment. These additional stops may alsoact as backup stops in the case of a primary stop failure.

For example and with reference to FIGS. 21-24, one embodiment of thepresent invention contemplates the use of protrusions and cut-outs toact as secondary stops. As seen in FIGS. 21 and 22, central core member140 may contain protrusions 141 and 143 disposed along lower partiallycylindrical surface 144 of core member 140. Protrusions 141 and 143, inthis example, are placed along the sides of the cylindrical surface andprotrude outward from sidewalls 145 and 146 of lower cylindrical surface144 of central core member 140. Protrusions 141 and 143 may also beconfigured with angled surfaces 147 and 148 as seen in FIG. 21. Finally,as seen in FIGS. 21 and 22, protrusions 141 and 143 are placed centrallyalong the radius of lower partially cylindrical surface 144.

With continuing reference to the above FIGS. 21 and 22 and theircorresponding description, lower endplate 150 may further be configuredto receive protrusion 141 and 143 of core member 140. As seen in FIGS.23 and 24, side walls 151 and 152 of upper surface 153 of lower endplate150 are configured with notches or cut-outs 154 and 155 that receiveprotrusions 141 and 143 of central core 140. Notches 154 and 155 allowcentral core element 140 to be placed onto lower endplate 150. Notches154 and 155 may be designed to allow placement of core element 140substantially from only one angle, i.e. when core element 140 is loweredonto lower endplate 150 and core element 140 is in a neutral or parallelposition relative to endplate 150.

Once inserted, movement of core element 140 relative to endplate 150 isprovided by forming grooves 156 and 158 within the sidewalls 151 and 152of partially cylindrical surface 157 of lower endplate 150. Accordingly,protrusions 141 and 143 ride within grooves 156 and 158, respectively,during flexion and extension. Grooves 156 and 158 are further designedto match or mate with the specific configuration of protrusions 141 and143. As seen in FIG. 21, protrusions 141 and 143 comprise angledsurfaces 147 and 148. Accordingly, as seen in FIG. 23, grooves 156 and158 are configured to mate with angled surfaces 147 and 148 ofprotrusions 141 and 143. In this particular embodiment, as core element140 moves relative to lower endplate 150 along their respectivearticulating surfaces, protrusions 141 and 143 travel within grooves 156and 158 respectively along the partially cylindrical lower surface 157of the lower endplate 150.

Grooves 156 and 158 may act as stops to limit flexion and extension. Inthis embodiment of the present invention, grooves 156 and 158 aredesigned with endpoints (two endpoints within each groove) to interactwith protrusions 141 and 143. Accordingly, in one embodiment, grooves156 and 158 do not run the entire length of partially cylindrical lowersurface 157. While any number of variations may be provided, in oneembodiment, the length of grooves 156 and 158 allow for a range ofmovement that substantially matches the range of movement allowed for bystop 135 and interacting surfaces 125 and 126 of FIG. 18. In thismanner, the prosthetic disc contains two mechanisms by which rotation islimited, while each mechanism constrains the device in the same degree.Alternatively, grooves 156 and 158 may be designed such that movement ofcore element 140 is limited less than what is allowed by stops 135 andinteracting surfaces 125 and 126 of FIG. 18. In this manner, thelimiting mechanism of grooves 156 and 158 and protrusions 141 and 143act as secondary or back-up stops in case of primary stop failure.

Another aspect of protrusions 141 and 143 and grooves 156 and 158contemplated by the present invention is their use in limitingseparation of core element 140 from lower endplate 150. For example,angled surfaces 147 and 148 of protrusions 141 and 143 may interact withthe matching surfaces of grooves 156 and 158. Accordingly, any forceacting to separate core element 140 from lower endplate 150, is resistedby the interaction between protrusions 141 and 143 and theircorresponding grooves 156 and 158. In this manner, core element 140 canbe secured from separation from lower endplate 150.

In an alternate embodiment, the second endplate may be configured withcut-outs rather than grooves. In this embodiment of the presentinvention, the cut-outs are configured to receive the tabs orprotrusions of the core element. Rather than riding within grooves asdiscussed above, the protrusions are sized smaller than the cut-outs.Accordingly, during articulation between the surfaces of the coreelement and the second endplate, the protrusions move within the spacescreated by the cut-outs. Depending on the size of the cut-outs, theprotrusions may abut or come into contact with the sidewalls of thecut-outs, thus limiting movement of the core element relative to thesecond endplate. By varying the size of the cut-outs and protrusions,varying degrees of articulation may be achieved in different prostheticdisc designs. As with the discussion relating to the protrusions andgrooves described above, the protrusions and cut-outs may serve asprimary or secondary stops.

In an alternate embodiment and with reference to FIGS. 21 to 24A, thepartially cylindrical surface 157 of second endplate 150 may be bound bysidewalls 149 and 159 that are arcuate in shape. Additionally, the widthW.sub.1 of the partially cylindrical surface 144 of core 140 may beslightly smaller than width W.sub.2 of the partially cylindrical surface157 of second endplate 150. Also, as seen in this embodiment, thesidewalls 145 and 146 of the core member may be linear or straight. Inthis embodiment, in addition to the anterior-posterior rotation providedby the partially cylindrical surfaces of the core member 140 and secondendplate 150, this configuration also allows rotation of the core member140 relative to the second endplate 150 in the coronal plane. As one ofskill in the art would understand, rotation of the core member 140relative to second endplate 150 is limited by the interaction ofsidewalls 145 and 146 of the core member and sidewalls 149 and 159 ofthe second endplate. Additionally, W.sub.1 and W.sub.2 may be varied tocontrol the range or amount of rotation in the coronal plane.

As discussed previously, the endplates of the prosthetic disc may alsobe configured to engage more securely with the vertebral bodies thatthey contact. For instance, one or more raised ridges or keels mayextend at least partially into the endplate of the vertebral body. Thevertebral body likewise may be prepared by cutting a similar number ofgrooves or channels that will receive the keels. The grooves or channelsmay help guide the assembly into proper position in the treated area.This feature may be particularly beneficial when a certain orientationof the assembly relative to the vertebral body is desired.

In one embodiment of the present invention, the upper and lower portionsof a disc assembly may be configured with a keel that can engage with orcontact a neighboring vertebral body. One advantage of providing a keelis that it may be used to guide the assembly into position duringinsertion into a treated area of the spine. A channel or groove may becut out of a vertebral body next. Then, a physician may insert theassembly into the vertebral body so that the keel slides in the grooveor channel. The keel and grove may be substantially linear or straight,or alternatively, may be curved or arched so that the assembly rotatesand slides into position. The ridges or keels and corresponding channelsor grooves also may be straight or curved to match the desired insertionpath of the assembly. The grooves or channels formed in a vertebral bodymay help achieve the proper orientation and distance of the assembliesand provide for a secure anchoring of the endplate.

The cross-sectional profile of the keel may have different shapes. Forinstance, the cross-sectional profile of the keel may have the shape ofa wedge, a truncated wedge, a rectangle, or a square. The channel orgroove may be cut to have a cross-sectional profile correspondingapproximately to the shape of the keel. One advantage of the keel havinga truncated wedge cross-section is that a similarly shaped channel orgroove may ensure that the keel engages with the bony surface. Thisconfiguration may also provide increased resistance to expulsion of thedisc assembly.

In one embodiment, the cross-section of a ridge or keel may betriangular or have a truncated triangular shape. For example, as shownin FIG. 25, keel 160 is of a truncated triangular shape. The height ofkeel 160 may vary, but may be configured with sloped sides 162 and 164,as shown in FIG. 25, of about 10° from the longitudinal plane. Theheight of keel 160 may vary, but in general is designed to providesufficient contact area once inserted in the vertebral body to anchorendplate 165. The keel may be sized such that any groove or channel cutinto the vertebral body to accommodate the keel does not substantiallyimpact the structural integrity of the vertebral body.

The use of one or more keels may also increase bone to implant surfacecontact, thereby decreasing the likelihood that the assembly will shiftor move about of position. In one embodiment, the increase in surfacecontact may be about 5% or more, which in another embodiment theincrease may be about 15% or more.

Over time, it is believe that the stability of the disc assembly in thetreated area will further increase as bone growth engages with outersurfaces of the disc assembly. To facilitate this growth and increasedstability, all or part of the surfaces of the disc assembly that engagesor otherwise contacts bone may be treated to promote bony on-growth. Forinstance, titanium plasma may be provided on the keel or other portionsof the assembly to provide a matrix for bone growth. In addition, thekeel may be configured with notches, slots, or openings formed along itslength. As bone grows into these openings, the disc assembly will becomemore securely anchored in place.

As a disc assembly is first inserted into a treated area, it may need tobe repositioned, rotated or otherwise moved. For instance, repositioningthe disc assembly may be needed so that the keel can properly engagewith the channel or groove. As shown in FIGS. 26 and 27, keel 160 ofendplate 165 has an angled first leading edge 166. Additionally,endplate 165 may be configured with a second leading edge 167 that doesnot contain part of keel 160. Thus, in one embodiment the assembly canbe partially inserted into the treated area without keel 160 engagingwith or contacting the vertebral body. In one embodiment, the length ofsecond leading edge 167 is from about 1 mm to about 10 mm, while inanother embodiment second leading edge 167 is from about 2 mm to about 5mm. Alternatively, the length of second leading edge 167 may be fromabout 1% to about 20% of the length of the endplate 165 on which it isdisposed, or may be from about 2% to about 10%. The length of theendplate 165 may be determined by measuring the longitudinal centralaxis of the portion or endplate on which second leading edge 167 isdisposed.

In addition, referring again to FIG. 26, keel 160 may have first leadingedge 166 that is sloped or gradually increases in height. As seen inFIGS. 26 and 27, first leading edge 166 is sloped. Providing a rampedfirst leading edge 166 may aid in aligning and inserting keel 160 into agroove or channel formed in a vertebral body.

As mentioned previously, the keel of a disc assembly may be configuredto promote or permit bony ongrowth/ingrowth that may help hold the discassembly in place more securely. FIG. 26 further illustrates anembodiment of keel 160 having a plurality of slots or cuts 168 formed init. In FIG. 26, slots 168 may extend at an angle, such as from about 5°to about 40° off from a vertical direction, and more preferably fromabout 10° to about 30°. Keel 160 may have two or more, or even three ormore slots or cuts. One skilled in the art would appreciate that otherconfigurations may also be used to promote bony ongrowth/ingrowth thatmight help further secure the disc assembly in place. For instance, thekeel may have holes or apertures drilled into it, longitudinal orhorizontal slots may be formed, and the sidewalls of the keel may betextured with one or more grooves or channels that does not extend fullythrough the keel to the opposing sidewall.

In addition, the face of the keel that is first inserted into a grooveor channel may have a taper or chamfer. One potential advantage ofconfiguring a keel with a taper or chamfer on its face is that it mayassist in aligning the keel with the opening of the channel or groove.In addition, a chamfered or tapered face may help reduce drag forces andundesired cutting or gouging of the channel or groove as the keel ispushed toward its final position. As seen in FIG. 26, the face of keel160 is configured with a chamfer 169 to aid in the insertion of theprosthetic disc.

In alternative embodiments, the endplate or keel may be formed with oneor more interiorly threaded holes located on the anterior edge of theendplate or keel. These interiorly threaded holes may be configured forattachment with a tool. For example, the interiorly threaded holes maybe configured to engage with a tool having exteriorly threadedprotrusions. In this example, the protrusions of the tool would matewith the holes of the endplates and hence, the tool may engage, hold, orotherwise capture the endplates of the prosthetic disc assembly. Thetool may be designed to hold a single endplate or alternatively may bedesigned to hold both endplates and the core member as a unit and in arigid state. The tool may then be used to implant the prosthetic discinto the intervertebral space.

As discussed previously, prosthetic disc designs of the presentinvention may be comprised of a plurality of assemblies. For example,prosthetic disc designs may be comprised of two assemblies. In a twoassembly design, each assembly may be comprised of an upper endplate, acentral core member, and a lower endplate. One advantage to the twoassembly design is that it may be made of a smaller dimension and henceprovide for ease of implantation, which may be especially useful inminimally invasive implantation techniques. Another advantage of twoassembly designs is that they may be used where the vertebral bodies towhich the prosthetic discs will be in contact with are diseased orotherwise not susceptible to supporting a prosthetic disc with a centralkeel. A two assembly design subjects the outer sides of the vertebralbody to contact. Another advantage to a two assembly design is that itmay allow for increased contact surface area while minimizing the sizeof the prosthetic disc. Accordingly, if a surgeon wishes to minimizedistraction of organs and other body parts during an anterior approachto the vertebra, they may select a two assembly design which may notrequire as large of a surgical pathway. In this case, the surgeon mayimplant a first assembly through the surgical pathway and then implant asecond assembly through the surgical pathway. Accordingly, a surgeon maybe able to minimize the size of a surgical pathway to a size just largeenough to implant one assembly, yet maintain the advantage of implantinga prosthetic disc that has a large contact surface area.

A plurality of disc assemblies having varying heights, widths, lengths,and ranges of translation and rotation capability may be provided in akit to a physician so that the final selection of the proper discassembly can be made during the surgical procedure. For instance, aplurality of disc assemblies may be provided having disc heights varyingfrom about 5 mm to about 25 mm. In one embodiment, the disc heights maydiffer by a uniform increment, such as differing by about 1 mm or byabout 1.5 mm within a range.

Likewise, the length of the disc assembly may be varied to accommodatedifferent anatomies. For instance, disc assemblies may have longitudinalaxes that range from about 8 mm to about 35 mm. Incremental changes inthe length of the assemblies may also be provided in a kit, such as byproviding disc assemblies of different lengths in 2 mm increments. Inanother embodiment, a plurality of assemblies may have at least 2different lengths that differ by more than about 3 mm. For instance, oneset of disc assemblies may have a length of about 25 mm, while anotherset is about 28 mm in length. The length of the disc assembly preferablymay be selected to maximize implant/endplate contact area.

A plurality of assemblies may also be provided with differing ranges ofaxial rotation. For instance, one or more assemblies may have norestriction on rotational movement, or may have stops or other devicesthat prevent rotation only after the rotation has exceeded the range ofmotion of a natural, healthy spine.

Other disc assemblies of the present invention may permit a range ofaxial rotation in one direction, but restrict it in the oppositedirection. In other words, a disc assembly of this embodiment may permitlimited disc rotation so that a patient may rotate or turn their body toone side or in one direction, but not in the other. For example, a discassembly may allow rotation or movement between a 0° position, where thespine is not rotated or turned, to up to about 5°, up to about 8°, up toabout 10°, or up to about 15° in one direction only.

Various instruments and tools may be used to implant anterior prostheticdiscs disclosed. For example, in one embodiment, the prosthetic discsare implanted using a distractor. In this embodiment, the adjacentvertebra are distracted and the damaged disc is removed. Whiledistracted, the distractor is placed into the intervertebral space. Thedistractor is designed to maintain spacing between the adjacentvertebrae and provided a space to insert the prosthetic disc. In oneembodiment, the distractor may have interior protrusions spaced toaccept and support the individual components of the prosthetic disc.Following insertion of the distractor, each individual component may beinserted into the distractor and held in position. The distractor maythen be removed and the force of the contracting vertebral bodiescompacts the components of the prosthetic disc into place.

Alternatively, a tool holder may be provided that is capable of holdingthe assembled components of the prosthetic disc. Accordingly, with orwithout the distractor in place, the surgeon may insert all componentsof the prosthetic disc with the tool holder into the intervertebralspace.

As described above, a cylindrical surface may be provided in a discassembly in addition to a second, curved surface corresponding to aportion of a sphere. One feature of this combination of surfaces is thatthe disc can permit translation between the upper vertebral body and thelower vertebral body.

In one embodiment, the disc is capable of permitting translation of upto about 1 mm in the anterior-posterior direction, while in anotherembodiment the disc is capable of translation of up to about 5 mm. Somedisc assemblies may permit even more translation, such as up to about 7mm or even up to about 10 mm. As described above, mechanical stops maybe provided to limit the range of motion of the disc assembly.

The various features and embodiments of the invention described hereinmay be used interchangeably with other features and embodiments.Finally, while it is apparent that the illustrative embodiments of theinvention herein disclosed fulfill the objectives stated above, it willbe appreciated that numerous modifications and other embodiments may bedevised by one of ordinary skill in the art. Accordingly, it will beunderstood that the appended claims are intended to cover all suchmodifications and embodiments which come within the spirit and scope ofthe present invention.

What is claimed is:
 1. An intervertebral prosthetic disc comprising: afirst endplate having a first surface configured to substantially engagewith a first vertebral body and a curved second surface, wherein thefirst endplate includes at least one stop; a second endplate having afirst surface configured to substantially engage with a second vertebralbody and a curved second surface configured to correspondingly engagethe curved second surface of the first endplate, wherein the secondendplate includes an opposing surface corresponding to each stop of thefirst endplate; wherein the first endplate and the second endplatecomplement each other to permit movement in a medial to lateraldirection and in an anterior to posterior direction.
 2. Theintervertebral prosthetic disc of claim 1, wherein the first surface ofthe first endplate contains a first keel.
 3. The intervertebralprosthetic disc of claim 2, wherein the first surface of the secondendplate includes a second keel.
 4. The intervertebral prosthetic discof claim 3, wherein the first keel is configured to engage a groove inthe first vertebral body.
 5. The intervertebral prosthetic disc of claim4, wherein the second keel is configured to engage with a groove in thesecond vertebral body.
 6. The intervertebral prosthetic disc of claim 5,wherein at least one of the first and second keels has at least one slotextending a length downwardly from a distal edge of the keel to the baseof the keel.
 7. The intervertebral prosthetic disc of claim 6, whereinthe at least one of the first and second keels includes a leading edgethat includes a chamfer for aligning and inserting the keel into thegroove in the first and second vertebral bodies.
 8. The intervertebralprosthetic disc of claim 7, wherein the keel comprises more than oneslot.
 9. The intervertebral prosthetic disc of claim 8, wherein thefirst endplate includes at least two stops.
 10. The intervertebralprosthetic disc of claim 9, wherein a resilient member is disposedbetween the at least two stops.
 11. An intervertebral prosthetic disccomprising: a first endplate having a first surface configured tosubstantially engage with a first vertebral body and a curved secondsurface, wherein the first endplate includes a first stop and a secondstop; a second endplate having a first surface configured tosubstantially engage with a second vertebral body and a curved secondsurface configured to correspondingly engage the curved second surfaceof the first endplate, wherein the second endplate includes a firstopposing surface configured to engage the first stop and a secondopposing surface to engage the second stop; wherein the first endplateand the second endplate complement each other to permit movement in amedial to lateral direction and in an anterior to posterior direction.12. The intervertebral prosthetic disc of claim 11, wherein the firstsurface of the first endplate contains a first keel.
 13. Theintervertebral prosthetic disc of claim 12, wherein the first surface ofthe second endplate includes a second keel.
 14. The intervertebralprosthetic disc of claim 13, wherein the first keel is configured toengage a groove in the first vertebral body.
 15. The intervertebralprosthetic disc of claim 14, wherein the second keel is configured toengage with a groove in the second vertebral body.
 16. Theintervertebral prosthetic disc of claim 15, wherein at least one of thefirst and second keels has at least one slot extending a lengthdownwardly from a distal edge of the keel to the base of the keel. 17.The intervertebral prosthetic disc of claim 16, wherein the at least oneof the first and second keels includes a leading edge that includes achamfer for aligning and inserting the keel into the groove in the firstand second vertebral bodies.
 18. The intervertebral prosthetic disc ofclaim 17, wherein the keel comprises more than one slot.
 19. Theintervertebral prosthetic disc of claim 18, wherein the first endplateincludes at least two stops.
 20. The intervertebral prosthetic disc ofclaim 19, wherein a resilient member is disposed between the at leasttwo stops.